The present invention relates to a method for reducing chromium (VI) and more particularly to a method using a chromium (VI)-resistant dissimilatory bacterial strain to reduce chromium (VI).
Hexavalent chromium (Cr(VI)) is a common toxic heavy metal pollutant in wastewaters and soils and elsewhere in natural ecosystems. Wastewaters containing Cr(VI) are produced by many industrial processes including chromium plating, metal cleaning and processing, wood preparation and alloy preparation. These wastewaters must be treated before being discharged into the environment. Chromium is also used in production of pigments, tanned leathers, fungicides, corrosion inhibitors in cooling water and drilling muds, wall paper, photographic films, magnetic tapes, printing inks and as a catalyst in the synthesis of many organic chemicals. Leakage, poor storage and improper disposal practices have released chromium into the environment in numerous locations causing contamination of ground and surface waters and of soils. These chromium-contaminated sites are subject to remediation requirements to reduce or eliminate the toxic levels of chromium.
As a result, efficient methods for cleaning up waste streams and for remedial clean up of contaminated sites are of critical importance. Conventional methods for removing toxic Cr(VI) ions include chemical reduction followed by precipitation, ion exchange and adsorption on coal activated carbon, alum, kaolinite, and fly ash. Most of these methods require high levels of energy or large quantities of chemical reagents. Other remediation methods involve the use of microorganisms such as the strain HO1 of Enterobacter cloacae which can anaerobically reduce Cr(VI) (Komori, K.; Kiyoshi, T.; and Hisao, O.; "Effects of Oxygen Stress on Chromate Reduction in Enterobacter cloacae Strain HO1", 1990, Journal of Fermentation and Bioengineering, 69(1):67-69). The Enterobacter strain HO1 can reduce Cr(VI) at levels of around 1-2 mM of potassium chromate, but levels above 5 mM are lethal to the bacteria. Some strains of Pseudomonas and Aeromonas which are capable of reducing Cr(VI) have also been identified.
Although some bacteria apparently use Cr(VI) as a terminal electron acceptor, it is not apparent that Cr(VI) reduction yields sufficient energy to support their anaerobic growth and reproduction. Although organisms such as Pseudomonas chromatophila (Lebedeva, E. V. and Lyalikova, N. N. "Reduction of crocoite by Pseudomonas chromatophila sp. nov.", 1979, Microbiology 48:517-522), Pseudomonas fluorescens (Bopp, L. H. and Erlich, H. L. "Chromate resistance and reduction in Pseudomonas fluorescens strain LB300", 1988, Arch. Microbiol. 150:426-431), and Enterobacter cloacae strain HO1 (Ohtake, H., Fujii, E., and Toda, K. "Bacterial reduction of hexavalent chromium: Kinetic aspects of chromate reduction by Enterobacter cloacae HO1", 1990, Biocatalysis 4:227-235; and Wang, P., Mori, T., Komori, K., Sasatsu, M., Toda, K. and Ohtake, H. "Isolation and characterization of an Enterobacter cloacae strain that reduces hexavalent chromium under anaerobic conditions", 1989, Appln. Environ. Microbiol. 55:1665-1669), can reduce Cr(VI) anaerobically, no evidence for Cr(VI)-dependent growth has been presented (Lovley, D. R. "Dissimilatory metal reduction", 1993, Ann. Rev. Microbiol. 47:263-291).
The application of Cr(VI)-reducing bacteria such as Enterobacter cloacae and Desulfovibrio vulgaris has been proposed as a potential means of treating Cr(VI)-containing waters and waste streams. However, D. vulgaris cannot grow with Cr(VI) as an electron acceptor, and thus treatment systems using this organism would require continual reinoculation. Further, E. cloacae requires a rich, expensive, heterotrophic medium in order to reduce Cr(VI), limiting the cost-effectiveness of its use in Cr(VI) treatment systems.
Shewanella alga strain BrY is an obligately respiratory, facultatively anaerobic bacterium which can grow anaerobically by coupling the oxidation of organic acids or H.sub.2 to the reduction of Fe(III), Mn(IV), U(VI), (Caccavo, Jr. F., R. P. Blakemore, R. P. and Lovley, D. R. "A hydrogen-oxidizing Fe(III)-reducing microorganism from the Great Bay Estuary, New Hampshire", 1992, Appl. Environ. Microbiol. 58:3211-3216), or Co(III)-EDTA. Further, strain BrY contains an electron transport chain and terminal reductase which can couple the oxidation of lactate or H.sub.2 to the reduction of Cr(VI) as well. However, all attempts to grow this organism with Cr(VI) as the sole terminal electron acceptor have heretofore been unsuccessful. Similar results have been observed with Desulfovibrio vulgaris (Lovley, D. R. and Phillips, E. J. P. "Reduction of chromate by Desulfovibrio vulgaris and its c3 cytochrome", 1993, Appl. Environ. Microbiol. 60:726-728).
Identification of a bacterial strain not only resistant to high levels of Cr(VI) but which could also use Cr(VI) as a terminal electron acceptor in a process yielding sufficient energy to sustain continuous growth in situ would be a valuable means of purifying Cr(VI)-containing soils, waters, and waste streams.